In recent years, a wireless apparatus such as a mobile phone and a smartphone comes to have a digital communication function, and data transmission is performed with high efficiency. When a wireless apparatus which sends data employs a multi-phase modulation system for data transmission, preferably an amplification characteristic of a transmission power amplifier is linearized to suppress a nonlinear distortion and to reduce leakage power to adjacent channels.
When an amplifier with low linearity is employed in an effort to improve power efficiency, it is preferable that a nonlinear distortion that occurs due to the low linearity be compensated. To this end, wireless apparatuses are provided with a distortion compensator that compensates for the nonlinear distortion of the power amplifier.
For example, a transmission signal is converted into two signals, i.e., an I signal and a Q signal, by an S/P (Serial/Parallel) converter. Then, the I signal and the Q signal are subjected to amplification processing. Thereafter, a part of the amplified signal is fed back as a feedback signal. The distortion compensator compares the I signal and the Q signal with the feedback signals, and calculates a distortion compensation coefficient so that a difference between the I signal/Q signal and the feedback signal is zero. Then, the distortion compensator performs distortion compensation by multiplying the I signal and Q signal, i.e., the converted transmission signal, with the distortion compensation coefficient. The distortion compensator compares the transmission signal before the distortion compensation with a feedback signal obtained by extracting a part of a signal amplified after the distortion compensation, and sequentially updates the distortion compensation coefficient so that the difference between these signals is zero. As to an example of such a conventional technique, see Japanese Laid-open Patent Publication No. 2001-189685, for example.
However, the conventional technique does not consider reducing a time taken for compensating the distortion of signals output from a plurality of amplifiers.
A wireless apparatus including a plurality of amplifiers may update the distortion compensation coefficient for each amplifier by switching between feedback signals from the amplifiers equally in a time-divisional manner, and comparing each feedback signal with a corresponding transmission signal.
Depending on the states of distortion in output signals from the plurality of amplifiers, a time taken for distortion compensation of the output signals can be long. For example, assume that distortion compensation is performed for two amplifiers; and an output signal from one amplifier experiences a relatively large distortion while an output signal from the other amplifier experiences no distortion. In this case, if the distortion compensation coefficients for the two amplifiers are updated equally in a time-divisional manner, an assigned time slot is not long enough for the signal experiencing a relatively large distortion for updating the distortion compensation coefficient, and hence, the update will be insufficient. On the other hand, the distortion compensation coefficient of the signal not experiencing a distortion is also updated, which is unnecessary. Consequently, as to the amplifiers as a whole, a time taken for the distortion compensation of the output signals may become long, because the distortion compensation of the output signal from the amplifier which experiences a relatively large distortion may take a long time until it finishes.